You can walk up and ask for the "cheapest ticket to get in," which often lands you in the Upper Reserve for around $15–$17.
On home game days, windows typically open 90 minutes before first pitch and stay open until roughly one hour after the game begins.
The biggest draw is avoiding the $10–$20+ processing fees tacked on by sites like SeatGeek or StubHub. Buying at the window often reduces fees to as little as $5 per ticket, or eliminates them entirely for "Standard" listings.